1. Filed of the Invention
The present invention is related to the production of hydrogen and/or carbon monoxide rich gases by steam reforming of hydrocarbons. In particular, the present invention involves a gold containing nickel reforming catalyst for use in the steam reforming of a hydrocarbon feedstock.
2. Description of the Related Art
In the known processes for the production of hydrogen and/or carbon monoxide rich gases, a mixture of hydrocarbons and steam and/or carbon dioxide is passed at elevated temperature and pressure through a reactor packed with a catalyst, mainly consisting of nickel as the active catalytic component.
Hydrocarbon feedstocks suitable for steam reforming are for instance natural gas, refinery off gases, propane, naphtha and liquified petroleum gases. Taking methane as an example, the reactions which take place can be represented by the following equations: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 (1) EQU CH.sub.4 +CO.sub.2 .fwdarw.2CO+2H.sub.2 (2) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 (3)
In addition to the reforming reactions (1) and (2), certain carbon forming reactions may occur as follows: EQU CH.sub.4 .fwdarw.C+2H.sub.2 (4) EQU 2CO.fwdarw.C+CO.sub.2 (5)
The carbon thereby formed is detrimental in several ways. It decreases the activity of the catalyst by blocking its active sites. Carbon formation may further cause spalling and pulverization of the catalyst particles, resulting in increasing pressure drop over the catalyst bed and hence discontinuation of the reforming operation due to clogging of the reactor.
It is well known to prevent carbon formation by increasing the steam to hydrocarbon ratio in the process gas or by reducing the molecular weight of the feedstock to reduce the potential for carbon formation.
However, increased steam to hydrocarbon ratio will lead to reduced yields of Co by shifting more carbon monoxide over to carbon dioxide. Thus, in order to maintain CO/H.sub.2 -production at the desired production rate, larger amounts of catalyst and feedstock are required, which depreciate the economy of the reforming process.
Various attempts have been made to overcome carbon deposition without affecting the optimum steam to hydrocarbon ratio.
A method of preventing carbon formation during the is reforming process for the preparation of reducing gases with a high reduction potential is mentioned in GB patent No. 2,015,027. By the disclosed process a feed gas rich in methane is reformed over a supported nickel catalyst in the presence of 2 to 10 ppm by volume sulphur or sulphur compounds in the feed. Thereby, the presence of sulphur counteracts carbon formation and ensures satisfactory reforming activity of the catalyst to form carbon monoxide and hydrogen.
Furthermore, several catalysts giving decreased carbon deposition have been suggested in the past. In general, the prior art catalysts having a suppression effect upon carbon deposition mainly consist of nickel with alkali promoter. Disadvantages of the alkali promoted catalysts are low activity and the mobility of the alkali metals, which cause migration and evaporation of the alkali promoter during the operation of the catalyst.
Catalysts free of alkali metals are suggested in U.S. Pat. No. 3,926,583, by which a nickel, iron or cobalt reforming catalyst is prepared by reducing a precursor comprising an intimate mixture of magnesium aluminum spinel with a mixed solid phase of the oxides of nickel, iron or obalt and in U.S. Pat. No. 3,791,993, disclosing a nickel, iron or cobalt, magnesium oxide reforming catalyst. Other promoters have been suggested in the art. U.S. Pat. No. 4,060,498 mentions a steam reforming process with a silver promoted nickel catalyst on a heat resistant oxide carrier. Furthermore, EP Patent No. 470,626 states the effect of Group IVa and Va metals suppressing the carbon formation. Amounts between 0.1 to 30% by weight calculated on the amount of metallic nickel, of germanium, tin, lead, arsenic, antimony and bismuth are included in the nickel containing catalyst.
Elements from Group Ib are not included in this patent, and in the literature, it is stated that addition of gold has no effect neither on carbon formation nor on the reforming activity (Mono and bi-metallic catalysts for steam reforming, Ph.D. Thesis by Isar-Ul Haque, University of New South Wales, 1990).
Recent STM studies (L. P. Nielsen et. al., Phys. Rev. Lett. 71 (1993) 754) have shown that gold may form an alloy in the surface of a nickel single crystal although these two elements are immiscible in the bulk. Furthermore, calculations using the density functional theory, predicts that addition of small amounts of gold will alter the reactivity of the neighbouring nickel atoms (P. Kratzer et al., J. Chem. Phys. 105 (13)(1996) 5595). Molecular beam studies of single crystals of nickel promoted with small amounts of gold have confirmed this (P. M. Holmblad et. al., J. Chem. Phys. 104 (1996) 7289).
Thus, it is the main object of this invention to prevent carbon deposition in the steam reforming of hydrocarbons.
We have now observed that addition of small amounts of gold to a nickel containing catalyst provides a catalyst with suppressed carbon deposition during steam reforming of hydrocarbons. Though gold decreases the catalytic activity of the nickel catalyst, the catalyst still provide sufficient activity for the steam reforming. EXAFS confirms that the gold is located at the nickel surface (Annual Report from HASYLAB, 1996).